Can you give more context?
Do you mean unique in a linguistic or in a political sense ore something else?
The sentence itself is strange.
"Begriff" and "Wort" are different concepts in linguistics, but seldom in coll. language.

If you mean "Begriff" as "Wort" - I do not understand the question.
If you mean it as concept, it is related to politic or culture and should be invariant when you transfer it to another language.
How I understand Frank's different translations"Vergangenheitsbewältigung ist ein Begriff, der fastnur die deutsche Sprache betrifft." - In this case it says that it is concerning almost only the German language - not another language - and almost nothing outside language.

"Vergangenheitsbewältigung ist ein Begriff, der eine ziemliche Eigenheit der deutschen Sprache ist." It is something special in German."Vergangenheitsbewältigung ist ein relativeinzigartiger Begriff der deutschen Sprache." there is (almost) no other concept in the German language with the same meaning.

I think the OP means that German is virtually the only language which can succinctly label this concept using a single word, and that in nearly all other languages you have to describe it instead, using several words, here perhaps "the ability to come to terms with the past".

I think the OP means that German is virtually the only language which can succinctly label this concept using a single word, and that in nearly all other languages you have to describe it instead, using several words, here perhaps "the ability to come to terms with the past".

Something can be unique, but it cannot be “relatively” unique. Es kann eigenartig sein, kann aber nicht “relativ“ eigenartig sein. Unique means there is only one.

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"Eigenartig" does not mean unique, it means strange. I think you meant to say "einzigartig".
While I understand that you want to say that uniqueness has no gradations, that nothing can be "more unique than" something else, that does not imply that something cannot be "relatively unique". It can, if "relatively" is used in the sense of "almost", though "unique" is then being abused to mean something like "rare". Something truly unique is so rare that there exists only one instance of it. Something relatively unique is so rare that there exist only a few instances.

That said, "relatively unique to" seems here to be used as indicating "relatively peculiar to".

Same Problem in German, I think. "Relativ einzigartig" does sound eigenartig, and it would perhaps be better to say "fast einzigartig".

“Einzigartig” is literally “one of a kind”. I do not think any speaker of English would say “relatively one of a kind”. Relative to what? “Unique” is the same word as Latin/German “unicum/Unikum”. I do not think any German speaker would say “ein relatives Unikum”. If you are not sure whether something is really unique you can of course always say “probably unique” or the like.

I think the OP means that German is virtually the only language which can succinctly label this concept using a single word, and that in nearly all other languages you have to describe it instead, using several words, here perhaps "the ability to come to terms with the past".